1. Field of the Invention
The present invention relates to a shift control device for a continuously variable transmission, in which a cruise-control accelerator-opening characteristic curve set on the basis of a requested torque and an engine rotation speed during cruise control is set along an equal horsepower line.
2. Description of the Related Art
There have been known vehicles equipped with cruise control devices. A cruise control device performs feedback control by detecting a difference (speed difference) between a vehicle speed set by a driver and an actual vehicle speed and controlling an engine output on the basis of the speed difference and a gear shift ratio of an automatic transmission so that the actual vehicle speed matches a target vehicle speed. For example, when a vehicle is running in a cruise control mode, if the vehicle reaches a road surface that requires a large running load, such as an uphill slope, the vehicle speed temporarily decreases. Therefore, the opening of a throttle valve is increased by performing engine control so that the engine output is increased. On the other hand, in shift control, since the opening of the throttle valve is greatly increased, the vehicle is accelerated by downshifting the automatic transmission so that the actual vehicle speed returns to the set vehicle speed.
As an example of shift control performed during a cruise control mode, Japanese Unexamined Patent Application Publication No. 2008-120268 discloses a technology of firstly obtaining a requested torque on the basis of the speed difference between the actual vehicle speed and the target vehicle speed, reverse-calculating an accelerator opening (cruise-control accelerator opening) on the basis of the requested torque and the engine rotation speed, obtaining a requested cruise-control accelerator opening from the cruise-control accelerator opening, and performing shift control on the basis of the requested cruise-control accelerator opening and the vehicle speed.
Furthermore, Japanese Unexamined Patent Application Publication No. 2008-120268 also discloses a technology for preventing gear hunting caused as a result of repetition of kick-down and downshifting. Specifically, gear hunting is prevented by limiting the cruise-control accelerator opening to an upper limit threshold value so that excessive shift operation is minimized when the cruise-control accelerator opening significantly fluctuates in response to a small change in the requested torque.
As an automatic transmission installed in a vehicle, there is known a continuously variable transmission (CVT) that can set the gear shift ratio in a continuous and stepless manner. Shift control in a typical CVT is performed by firstly setting a target primary rotation speed (i.e., a value proportional to the engine rotation speed) on the basis of the vehicle speed and the accelerator opening and then calculating a target gear shift ratio (secondary-rotation-speed/primary-rotation-speed) from the target primary rotation speed and an actual secondary rotation speed.
As mentioned above, because the gear shift ratio in the CVT is set in a continuous and stepless manner, there is substantially no hysteresis in response to a change in the accelerator opening. Therefore, gear hunting tends to occur excessively during cruise control, as compared with a multistage transmission.
FIG. 4 illustrates a shift-line map used in a CVT. As shown in FIG. 4, in the shift-line map, the accelerator opening is set in a region surrounded by a low-speed shift line LOW corresponding to a maximum target gear shift ratio (target primary rotation speed) and a high-speed shift line OD corresponding to a minimum gear shift ratio. The target primary rotation speed (≈target engine rotation speed) is determined on the basis of the current vehicle speed and the accelerator opening.
Specifically, because the rotation of an output shaft of the engine is decreased in speed by the CVT so as to become the wheel speed (vehicle speed), if the target primary rotation speed engine rotation speed) is set relative to the vehicle speed on this shift-line map, the gear shift ratio of the CVT can be determined. Therefore, since the target primary rotation speed increases with increasing accelerator opening when the vehicle is running at a constant vehicle speed, the gear shift ratio becomes a large value.
As shown in FIG. 4, in a region lower than or equal to a curve L1 that indicates that the vehicle speed is equal to a mid speed S1 (for example, 40 km/h) and the accelerator opening at that time is a small opening (for example, 20%), the target primary rotation speed does not significantly change even in response to a change in the accelerator opening. However, when the accelerator opening is increased, the target primary rotation speed is set to change significantly in curves L2 and L3 indicating intermediate openings (for example, 35% and 50%, respectively). Therefore, even when the vehicle is running at a constant vehicle speed, the target primary rotation speed significantly changes when the accelerator opening is between a small opening and an intermediate or large opening. Correspondingly, the CVT downshifts or upshifts so that the gear shift ratio matches the target gear shift ratio.
If the target primary rotation speed is set on the basis of the aforementioned cruise-control accelerator opening and the vehicle speed by referring to the aforementioned shift-line map during cruise control, the shift operation in the CVT becomes excessive in a region in which the target primary rotation speed significantly changes in response to a slight change in the cruise-control accelerator opening, causing gear hunting to occur readily.
As a countermeasure against this problem, an excessive shift operation during a transitional period can be suppressed by limiting the cruise-control accelerator opening to an upper limit threshold value, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-120268 mentioned above. However, this means that the shift operation is also limited for power-requiring running conditions, such as accelerating from a complete stop or running on an uphill slope. This is problematic in that the driver may feel a lack of output.